1. Field of the Invention
The present invention relates to a method of processing a silica glass fiber.
2. Description of the Related Art
Silica glass fibers have been utilized for transmitting ultraviolet rays (hereinafter referred to as UV) (especially excimer laser beam), for example, in a photomask for excimer laser lithography, in a light guide for transmitting UV used to irradiate UV-hardening resin, and in the fields of microfabrication, medical treatment and the like.
When a silica glass fiber is used for transmitting UV, for example, to irradiate a UV-hardening resin, transmission of UV with a shorter wavelength and higher power is required, so that the hardening time of the resin can be reduced. The short wavelength and high power of the UV must therefor be fully available.
However, when UV is transmitted through silica glass, structural defects in the silica glass, decrease the transmittance. The decrease of transmittance of silica glass becomes greater, as the wavelength of UV becomes shorter and its light power becomes higher. Therefore, when an excimer laser is used as a light source, the transmittance of silica glass becomes worse especially with KrF excimer laser (wavelength: 248 nm) to F2 excimer laser (wavelength: 157 nm) including ArF excimer laser (wavelength: 193 nm). The transmittance becomes worse when a laser of higher light power (one of various excimer lasers like KrF, ArF and F2) is used as opposed to when a lamp of lower light power (a halogen lamp, a deuterium discharge lamp and the like) is used as a light source.
In order to reduce the decrease of transmittance of silica glass due to UV irradiation, or to improve resistance of silica glass to UV, a technique of increasing the hydroxyl group content of silica glass has been proposed in the publication of Japanese Unexamined Patent Application Hei 4-342427, publication of Japanese Unexamined Patent Application Hei 4-342436, etc. However, when the hydroxyl group content is increased, the wavelength of UV absorption edge becomes longer, with a result that UV of short wavelength (especially, vacuum ultraviolet zone) cannot be transmitted.
The solution to this problem was provided by a method (disclosed in co-pending U.S. patent application Ser. No. 09/351,951) in which multiple structural defects are purposefully caused in silica glass by irradiating silica glass with UV, and in which the structural defects are removed by performing heat treatment simultaneously with or after the UV irradiation.
In applying the method disclosed in the co-pending U.S. patent application Ser. No. 09/351,951 to a silica glass fiber, there occurred the following problems.
When UV with high power is repeatedly irradiated through the end surface of a fiber to cause structural defects, deterioration occurs only at the irradiated end, and UV does not reach the other end. Therefore, only a short fiber can be processed and it is impossible to process a long fiber throughout length. Also cutting a long fiber into a plurality of short fibers (about 1 m, for example) and processing these fibers one by one leads to an increase of costs.
On the contrary, if a relatively long fiber is irradiated with a lower power, it can be processed, but that requires a substantially long processing time and thus is unsuitable for mass production
It is an object of the invention to provide a method of changing the properties of silica glass fiber (an improvement in resistance) to UV caused defects by UV irradiation and heat treatment, which is applicable to a long fiber and suitable for mass production, and in consequence achieves improved productivity and reduced cost.
To attain this and other objects, in the first aspect of the present invention, there is proposed a method of processing a silica glass fiber, in which multiple structural defects are purposefully caused in a silica glass fiber by irradiating the silica glass fiber with UV, and the structural defects are removed by performing heat treatment in order to improve the UV resistance of the silica glass fiber, the method comprising the steps of irradiating the UV through one end surface of the silica glass fiber and shifting the heating zone from the one end surface through which the UV is irradiated toward the other end surface.
In the second aspect of the present invention, there is proposed the method of processing a silica glass fiber in the first aspect of the present invention, in which a heat resistant coating is applied to the silica glass fiber.
In the third aspect of the present invention, there is proposed the method of processing a silica glass fiber in the second aspect of the present invention, in which the heat resistant coating is a metal coating.